Staphylococcus aureus is one of the most important causes of life-threatening bacterial infections in the world. This bacterium can colonize and invade virtually all tissues in the human body. The tremendous success of S. aureus as a pathogen is in part due to the expression of an arsenal of virulence factors. Among these, S. aureus produces several cytotoxins that are secreted into the extracellular milieu and have the potential to kill mammalian cells. In this application, we describe the identification and preliminary characterization of a novel staphylococcal cytotoxin that we have named leukocidin A/B (LukAB). LukAB is a pore-forming toxin that specifically and potently targets and kills host phagocytes, important cells of the immune system that play a major role in controlling staphylococcal infections. The aims of this application are to: (i) identify and characterize host molecules responsible for LukAB tropism towards phagocytes, (ii) define functional domains required for LukAB cytotoxicity, and (iii) define the contribution of LukAB to S. aureus pathogenesis. To accomplish these Aims we propose to employ a combination of genetic and biochemical approaches, together with tissue culture and animal models of infection. Understanding the molecular mechanism by which LukAB targets and kills host cells may pave the way for developing novel therapeutics targeting this potent toxin produced by one of the most significant microbial threats worldwide.